Ex-White House chief strategist Steve Bannon says President Trump's firing of the FBI's director was the biggest mistake in "modern political history".
Mr Bannon told CBS News if James Comey had not been sacked, a special counsel would not have been appointed to probe alleged Russian election meddling.
The former presidential adviser called the special counsel inquiry a "waste of time" and "a total and complete farce".
Mr Bannon was himself dismissed from his White House job last month.
Bannon should worry conservatives
By Anthony Zurcher, BBC News North America reporter
Steve Bannon says he's "going to war" with the Republican political establishment. For conservatives - and even President Donald Trump - that should be very concerning.
Mr Bannon seeks to tap into the same anti-Washington resentment that has fuelled the grass-roots Tea Party movement since the early days of the Obama presidency.
The Tea Party's contribution to the Republican cause, however, has been decidedly mixed.
While it helped sweep Mr Trump to the presidency, and brought new energy to a moribund political hierarchy, the scalps the movement claimed were as likely to come from the right as the left.
Tea Party primary challenges that produced less palatable candidates almost certainly denied Republicans a Senate majority in 2010, even as they took over the House of Representatives.
It gave Democrats surprise Senate wins in Missouri and Indiana in 2012, where long-time incumbent Richard Lugar was knocked off by a primary opponent later bested by the Democrats.
If Mr Bannon starts targeting Republican establishment stalwarts - and he's already gunning for incumbent Alabama Senator Luther Strange - the party's efforts to stave off a Democratic surge in next year's mid-term congressional elections, and maintaining their control of both chambers of Congress, could be seriously compromised.
But he vowed in an interview with CBS' 60 Minutes programme broadcast on Sunday night to be Mr Trump's "wingman outside for the entire time" in office.
Mr Bannon has returned to his previous role as chairman of Breitbart News, a right-wing outlet.
A keen student of history, he apparently rates Mr Comey's ousting as a presidential error even more egregious than Bill Clinton's liaison with Monica Lewinsky, Ronald Reagan's Iran-contra affair or Richard Nixon's Watergate scandal.
Mr Bannon told CBS: "So the firing of James Comey was the biggest mistake in modern political history."
"I don't think there's any doubt that if James Comey had not been fired, we would not have a special counsel, yes," he added.
"We would not have the Mueller investigation," he said in a reference to special counsel Robert Mueller, a former FBI director, "and the breadth that clearly Mr Mueller is going for."
Mr Mueller was appointed by the US deputy attorney general after President Trump stunned Washington by firing Mr Comey on 9 May.
The special counsel's team is reportedly investigating the president's decision to fire Mr Comey.
"Russian collusion is a farce," Mr Bannon said of suggestions that Trump campaign aides collaborated with a Kremlin plot to defeat Hillary Clinton in last year's election.
But he said he does not believe the special counsel should be fired.
The former White House chief strategist also turned his fire on Republican congressional leaders Paul Ryan and Mitch McConnell.
He accused them of "trying to nullify the 2016 election".
"They do not want Donald Trump's populist, economic nationalist agenda to be implemented," Mr Bannon told 60 Minutes.
"It's obvious as night follows day."